This invention relates generally to the handling of objects over the side of a ship at sea and, more particularly, to an improved deck handling system in which the motion of a surface support ship is isolated from a payload suspended in the ocean by a lifting cable.
The handling of objects over the side of the ship at sea is historically a difficult problem. The motion of the ship can induce large dynamic loads on the lift line and parting of lines during operations such as salvage recovery, undersea work vehicle operations and even basic moorings is not uncommon. Deck handling systems having active or passive motion compensation have been built but these have always involved great expense and usually provided poor reliability. The devices are very complicated mechanically and require a major modification to the ship to provide proper footings for them. They usually include a motion-damped over-the-side boom with the lift line winch or at least a traction unit mounted directly on the device, which must swivel and be capable of locking to allow handling the load over the side. By their nature they have very close, definite limits on their vertical travel. They are essentially fixed installations on the ship and are not easily disassembled for transfer to other vessels or for air shipment to various parts of the world. A similar technology exists in the many motion-compensation and tension-limiting systems used by the oil industry for drill strings, but these systems are not directly applicable to over-the-side handling. Partial solution to this difficult problem has been provided in the Linked-Spar Motion-Compensated Lifting System disclosed in U.S. Pat. No. 4,280,430 issued July 28, 1981. Nevertheless, some problems in connecting and non-destructively disconnecting the payload and its supporting buoy as well as limitations in isolating motion of the ship from the payload due to changing sea conditions have continued.